regex - sed - Commenting a line matching a specific string AND that is not already commented out

Question

I have the following test file

AAA
BBB
CCC

Using the following sed I can comment out the BBB line.

# sed -e '/BBB/s/^/#/g' -i file

I'd like to only comment out the line if it does not already has a # at the begining.

# sed -e '/^#/! /BBB/s/^/#/g' file

sed: -e expression #1, char 7: unknown command: `/'

Any ideas how I can achieve this?

Answer 1

Assuming you don't have any lines with multiple #s this would work:

sed -e '/BBB/ s/^#*/#/' -i file

Note: you don't need /g since you are doing at most one substitution per line.